diff --git a/internal/usecase/repairer.go b/internal/usecase/repairer.go
index aa83500..b0ed15c 100644
--- a/internal/usecase/repairer.go
+++ b/internal/usecase/repairer.go
@@ -132,7 +132,8 @@ func formatRepairDiagnostics(errors []string) string {
 		if len(encoded) > 1 {
 			separator = 1
 		}
-		if len(encoded)+separator+len(entry)+1+len(omission)+1 <= maxRepairDiagnosticBytes {
+		available := maxRepairDiagnosticBytes - len(encoded) - separator - 1 - len(omission) - 1
+		if len(entry) <= available {
 			if separator != 0 {
 				encoded = append(encoded, ',')
 			}
@@ -140,7 +141,9 @@ func formatRepairDiagnostics(errors []string) string {
 			continue
 		}
 
-		available := maxRepairDiagnosticBytes - len(encoded) - separator - 1 - len(omission) - 1
+		if available < len(`""`) {
+			break
+		}
 		if separator != 0 {
 			encoded = append(encoded, ',')
 		}
@@ -157,17 +160,24 @@ func formatRepairDiagnostics(errors []string) string {
 
 func truncateDiagnosticJSONValue(value string, maxBytes int) []byte {
 	if maxBytes < len(`""`) {
-		return []byte(`""`)
+		return nil
 	}
 
-	low, high := 0, len(value)
+	boundaries := []int{0}
+	for end := 0; end < len(value); {
+		_, size := utf8.DecodeRuneInString(value[end:])
+		end += size
+		if end+len(`""`) > maxBytes {
+			break
+		}
+		boundaries = append(boundaries, end)
+	}
+
+	low, high := 0, len(boundaries)-1
 	best := []byte(`""`)
 	for low <= high {
 		mid := low + (high-low)/2
-		for mid > 0 && mid < len(value) && !utf8.RuneStart(value[mid]) {
-			mid--
-		}
-		candidate, _ := json.Marshal(value[:mid])
+		candidate, _ := json.Marshal(value[:boundaries[mid]])
 		if len(candidate) <= maxBytes {
 			best = candidate
 			low = mid + 1
diff --git a/internal/usecase/repairer_test.go b/internal/usecase/repairer_test.go
index f575bf3..927fd12 100644
--- a/internal/usecase/repairer_test.go
+++ b/internal/usecase/repairer_test.go
@@ -234,6 +234,23 @@ func TestFormatRepairDiagnosticsBoundsAndPreservesData(t *testing.T) {
 				}
 			},
 		},
+		{
+			name: "no room for partial diagnostic",
+			errors: func() []string {
+				omission, _ := json.Marshal(omittedRepairDiagnostics)
+				return []string{
+					strings.Repeat("x", maxRepairDiagnosticBytes-len(omission)-5),
+					strings.Repeat("y", 128),
+				}
+			}(),
+			wantOmission: true,
+			check: func(t *testing.T, got []string) {
+				t.Helper()
+				if len(got) != 2 || got[1] != omittedRepairDiagnostics {
+					t.Fatalf("diagnostics = %#v", got)
+				}
+			},
+		},
 		{
 			name:   "invalid UTF-8",
 			errors: []string{"broken\xffinput"},
diff --git a/internal/usecase/runner_test.go b/internal/usecase/runner_test.go
index c9c5dad..91e1736 100644
--- a/internal/usecase/runner_test.go
+++ b/internal/usecase/runner_test.go
@@ -295,8 +295,10 @@ func (c *controlledRepairLLM) Generate(
 	ctx context.Context,
 	req domain.GenerateRequest,
 ) (*domain.GenerateResponse, error) {
-	isRepair := len(req.Prompt.Messages) > 0 &&
-		strings.HasPrefix(req.Prompt.Messages[0].Content, "You repair invalid JSON")
+	messages := req.Prompt.Messages
+	isRepair := len(messages) >= 2 &&
+		messages[len(messages)-2].Role == "assistant" &&
+		messages[len(messages)-1].Role == "user"
 	c.mu.Lock()
 	c.calls++
 	c.active++